Low power lasers are used to read and write data on the data side of optical media, such as compact discs (CDs), digital versatile discs (DVDs), and the like. Typically, a laser beam writes various types of data on a data side of a disc while the disc is rotating. Data may be recorded by making marks on the disc, representing binary digits. Various data writing strategies have been employed to create desirably shaped marks that are precisely positioned and that have sharp edges to enable detection.
Labels on such optical discs are typically used to provide descriptive human-readable indicia of the data content recorded on the disc, as well as illustrations, artwork, and the like. Such descriptions and images are generally handwritten, affixed, or marked on the side of the disc opposite the data side. Recently, apparatus and methods have been developed with the ability to generate an image or label on the non-data side (the “label side”) of an optical disc using a low power laser, such as the same laser that is employed to read and write digital or electronic data on the data side of the disc. For example, see U.S. Patent Application Publication No. 2003/0108708, Anderson, et al. which is commonly assigned with the present application.
In marking the data side of a disc using a laser in an optical disc drive, high importance is typically placed on creating marks having sizes, shapes, edge sharpness, and placement within precise tolerances that are generally required for accurately reading and writing digital data. However, a comparatively lower degree of precision is required to create marks for producing an image on the label side of the disc that is satisfactory to the visual perception of an unaided human eye. Accordingly, in marking a label side of a disc using a laser in an optical disc drive, it is desirable to optimize the writing process for other considerations, such as enhanced visible reflectivity or optical density of marks.
Conventional optical disc drives have a focusing servo incorporating a focus actuator. The focus actuator moves an objective lens in a Z-axis direction relative to the disc, to generally maintain the lens at a constant optimal focus position, sometimes called “best focus” position. The “best focus” position is typically optimized for reading and writing data on the data side of the disc, such as by selecting a default position where the laser beam is focused on a spot at a layer of recording material below the surface of the data side. The use of a defocused spot for writing disc labels is described by Anderson, et al., in U.S. patent application Ser. No. 10/732,047, filed Dec. 9, 2003, which is commonly assigned with the present application. By defocusing a laser spot size rather than using a focused laser spot, a larger marking spot can be achieved on optically labeled disc media without a sacrifice in speed.
For purposes of such label marking optimization, it may be useful to measure the reflectivity of marks on the label side of the disc. Such measurements may be done, for example, by measuring laser light generated by the writing laser and reflected from the marked surface. However, in marking the label side of the disc, using the technology described in the above-referenced U.S. Patent Application Publication No. 2003/0108708,the chemistry on the label is optimized to absorb as much light as possible at the wavelength of the writing laser beam. As a result, there is very little reflection of light at the frequency of the writing laser, and the change in reflectivity between written and unwritten areas is very small, resulting in a low signal-to-noise ratio in the reflectivity measurement. Accordingly, the use of a writing laser to also generate light for measuring the reflectivity of the written mark may not yield satisfactory results.